Naphtho[2,3-<i>a</i>]pyrene Thin Films – H, I, or J? Aggregate Alphabet Soup Served over Triplet Pair States

Photon upconversion in the solid state has the potential to improve existing solar and infrared imaging technologies due to its achievable efficiency at low power thresholds. However, despite considerable advancements in solution-phase upconversion, expanding the library of potential solid-state annihilators and developing a fundamental understanding of their solid-state behaviors remains challenging due to intermolecular coupling affecting the underlying energy landscape. Naphtho[2,3-a]pyrene has shown promise as a suitable solid-state annihilator. However, the origin of its multiple underlying emissive features remains unknown. To this point, here, we investigate NaPy/poly(methyl methacrylate) thin films at varying concentrations to tune the intermolecular coupling strength to determine its photophysical properties at a range of temperatures between 300–50 K. The results suggest that the multiple emissive features present in the NaPy thin film emission at room temperature arise from a multidimensional I-aggregate (520 nm), an excimer (550 nm), and a strongly coupled J-dimer (620 nm). In addition, we find that at low temperatures, the emission spectrum is dominated by direct emission from the 1(TT) state.